This invention relates generally to electrochemical fuel cells. This invention relates more particularly to a fuel cell that uses an amine-based fuel and an oxidant solution to yield high specific energy and power density.
Fuel cells are well-known electrochemical devices that enable the conversion of the chemical energy of fuels directly into electrical energy, thereby avoiding the Carnot cycle limitations and loss of efficiency associated with combustion-related engines. Hydrogen-oxygen fuel cells can be 50%-65% efficient in practice. This is far higher than typical values for internal combustion engines.
Over the past four decades, several different types of hydrogen fuel cells have been developed for space and terrestrial applications. These various types of fuel cells are typically categorized by the electrolyte they use. Examples of such contemporary fuel cells include the alkaline fuel cell (AFC), the phosphoric acid fuel cell (PAFC), the proton exchange membrane fuel cell (PEMFC), the molten carbonate fuel cell (MCFC) and the solid oxide fuel cell (SOFC). Another type of proton exchange fuel cell currently being developed is the direct methanol fuel cell (DMFC), which uses methanol as the fuel. Although such contemporary fuel cells have proven generally useful for their intended purposes, they all suffer from inherent deficiencies which detract from their utility and desirability, as discussed below.
Alkaline fuel cells (AFC) are capable of good power density. However, AFC""s have low tolerance for even very low concentrations of carbon dioxide, which gets absorbed by the electrolyte to form carbonates. This undesirable process inherently limits fuel cell operation to pure hydrogen and oxygen gases. AFCs exhibit good oxygen electrode performance, especially at high alkali concentrations. The active ionic species is the hydroxide ion (OHxe2x88x92). The upper limit of temperature is about 260xc2x0 C. AFCs can use non-noble metal catalysts. However, in automotive applications and the like, it is impractical to eliminate carbon dioxide completely from the processed hydrocarbon fuel, thus removing the AFC from transportation and similar applications. Additionally, for stationary power generation, unless a source for pure gases is available, AFCs are not deemed viable.
The phosphoric acid fuel cell (PAFC) can use ambient air and processed hydrocarbon fuel. However, PAFCs have only modest power density. Further, PAFCs cannot generate power at ambient temperature, but rather must be preheated to at least 100xc2x0 C. before power can be drawn. This is due to limitations of electrolytic conductance at lower temperatures. Further, once at operating temperatures (typically around 200xc2x0 C.), PAFCs must be kept below 0.8 volt per cell to prevent corrosion of cell components. This limitation makes it impractical to keep the fuel cell at open circuit (idle) for extended periods when hot. PAFCs, because of their high operating temperatures, can accept up to 1% carbon monoxide in the fuel stream. The use of an acid electrolyte limits the choice of electrocatalysts to noble metals. The active ionic species is the hydronium ion (H3O+).
Proton exchange membrane fuel cells (PEMFC) have been subject to significant research activity during recent years. The PEMFC uses an immobilized electrolyte membrane, a fully fluorinated Teflon-based polymeric material, which exhibits protonic conductance. A typical membrane produced by E.I. Dupont de Nemours has the generic name Nafion(copyright). It has a fluoropolymer backbone, upon which sulfonic acid groups are chemically bonded. The Nafion membranes exhibit exceptionally high chemical and thermal stability up to temperatures of 125xc2x0 C. Partially fluorinated membranes are also presently being investigated. PEMFCs offer modest power density, with low system weight, and volume. Because of the intrinsic nature of the materials used, low temperature operation of 80xc2x0 C. is possible. The cell is also able to sustain operation at high current density. This allows fast start capability and facilitates the construction of a compact, lightweight cell.
However, the low temperature of operation makes the use of noble electrocatalysts imperative, as well as increasing the fuel cell""s sensitivity to carbon monoxide poisoning. Only a few parts per million (ppm) of carbon monoxide can be tolerated by the electrocatalyst at 80xc2x0 C. A specific problem associated with the PEMFC has been the need to hydrate the membrane continuously to allow protonic diffusion by the hydronium ion (H3O+), which is the active ionic species.
A variation of the PEMFC, which has attracted recent attention, is the direct methanol fuel cell (DMFC). Methanol is the only practical carbonaceous fuel with good electrochemical reactivity at the fuel cell anode. Taking advantage of this reactivity, the DMFC can be almost as simple as a hydrogen-air fuel cell, while being able to use a readily stored, inexpensive fuel. However, the DMFC has been handicapped by two major problems: poor current density and rapid diffusion of methanol through the proton membrane to the air electrode. This methanol crossover problem results in short-circuit oxidation of methanol, thereby undesirably reducing both fuel utilization and electric output. Recent research activity has concentrated on improving methanol anode activity and possible approaches to reducing the crossover problem.
The molten carbonate fuel cell (MCFC) is often referred to as a second-generation fuel cell. The MCFC operates at a significantly higher temperature, typically around 650xc2x0 C. The electrolyte is usually a mixture of lithium aluminate and alkali carbonates. The active ionic species is the carbonate ion (CO3xe2x88x92). The higher operating temperatures of MCFCs allow achievement of higher system efficiencies and greater flexibility in the use of available fuels, as well as feasibility of cogeneration. However, the high temperatures place severe demands on the corrosion stability and life of cell components in the aggressive molten carbonate environment. Electrolyte management is critical in the MCFC. Several competing processes cause redistribution of the molten carbonate ion. The MCFC uses nickel-based anodes and nickel oxide cathodes. Structural stability of the anodes and dissolution of the NiO cathodes in the carbonate salt have been major problems. Cell performance has typically been at modest power densities.
Solid oxide fuel cells (SOFC) have recently attracted interest as viable high temperature fuel cells. The SOFC has no liquid electrolyte, with its attendant problems of corrosion and electrolyte management. The operating temperature of 650xc2x0 C.-1000xc2x0 C. allows internal fuel reforming, rapid kinetics with non-noble catalysts and produces heat for cogeneration. The active ionic species is the oxygen ion (Oxe2x88x92). The solid state character of the SOFC permits no restriction on the cell configuration. Both tubular and flat plate designs are being developed. Cost reduction of cell components and simplification of the manufacturing process are an important focus of ongoing development. High temperature (1000xc2x0 C.) and intermediate temperature (650xc2x0 C.) ceramics with the requisite conductivity are being investigated. The thermodynamic efficiency of SOFC is less than that of the MCFC and the PAFC, but the higher temperature is beneficial in reducing polarization.
A global revolution is taking place in telecommunications, information systems and the electric power industries, with important implications in the defense sector. The power source is the weak link in these industries, especially in distributed power generation. There is a need for critical, enabling power technologies to cater to these changes. Advanced batteries and other energy storage devices are needed for portable wireless electronics such as laptops, cellular phones, mission-critical instruments, low earth orbital satellites and micro-reconnaissance systems.
Several applications have been identified in the defense sector, which need new compact, high energy density power sources. Some applications include high rate primary and secondary batteries, semi-fuel cells and fuel cells for high speed underwater vehicles and torpedoes, and low rate rechargeable energy systems for long endurance missions in unmanned underwater vehicles (UUVs). Smart unmanned air vehicles and micro-vehicles for reconnaissance and stealth technology systems need compact power sources with small thermal and acoustic signatures. Distributed high quality electric power for supporting small groups of soldiers is needed, especially if such power can also facilitate cogeneration of heat and/or hot water.
Some of the alternatives for such portable power sources are planar solid oxide fuel cells (SOFC), direct methanol fuel cells (DMFC), thermophotovoltaics, alkali metal thermal to electric converters, small turbine engines, advanced rechargeable batteries, ultracapacitors and flywheels. These power technologies would also be of commercial significance in the convergence of digital, broadband, multimedia communications and computing; in mobile electronics; in dispersed fiber or wireless low earth orbit satellite based transmission networks; and in electric power for power quality sensitive industries. There exists a premium on high quality and reliability in electric power in these industries, from the milliwatt to the kilowatt level.
During the last decade, several new developments have evolved in stand-alone premium power. These developments represent radical departures from previous practice. They include novel thin film materials and membranes, used in unique multilayer device designs in solar cells, proton exchange membrane and solid oxide fuel cells, lithium-ion batteries and innovative intercalated anodes and cathodes. There has been a movement away from chemically dissimilar redox couples, limited by the periodic table, to new batteries based on single ion conductors in nickel metal hydride and lithium ion rocking chair batteries. New polymer and ceramic membranes promise to revolutionize charge transfer in solid state fuel cells and batteries, allowing use of production techniques from the electronic chip manufacturing sector.